Article missed one of the distinct possibilities. Now that Putin controls the legislative branch, he has enough votes there to change the constitution to allow him to run for President again in March. I see that as more probable than the Prime Minister scenario.
So what's he going to do? I don't know, but he's promised not to run for a third term, and I expect him to keep that promise in the literal sense.
That doesn't mean he won't figure out a way to keep the promise and still retain power.